CSCI-1680Security
Based on lecture notes by Scott Shenker and Mike Freedman
Andrew Ferguson
Today’s Lecture
• Classes of attacks• Basic security requirements• Simple cryptographic methods• Cryptographic toolkit (Hash, Digital
Signature, …)• DNSSec• Certificate Authorities• SSL / HTTPS
Basic Requirements for Secure Communication
• Availability: Will the network deliver data?– Infrastructure compromise, DDoS
• Authentication: Who is this actor?– Spoofing, phishing
• Integrity: Do messages arrive in original form?
• Confidentiality: Can adversary read the data?– Sniffing, man-in-the-middle
• Provenance: Who is responsible for this data?– Forging responses, denying responsibility– Not who sent the data, but who created it
Other Desirable Security Properties
• Authorization: is actor allowed to do this action?– Access controls
• Accountability/Attribution: who did this activity?• Audit/Forensics: what occurred in the past?
– A broader notion of accountability/attribution• Appropriate use: is action consistent with policy?
– E.g., no spam; no games during business hours; etc.• Freedom from traffic analysis: can someone tell
when I am sending and to whom?• Anonymity: can someone tell I sent this packet?
Internet’s Design: Insecure• Designed for simplicity in a naïve
era• “On by default” design• Readily available zombie machines• Attacks look like normal traffic• Internet’s federated operation
obstructs cooperation for diagnosis/mitigation
Eavesdropping - Message Interception (Attack on Confidentiality)
• Unauthorized access to information• Packet sniffers and wiretappers• Illicit copying of files and programs
A B
Eavesdropper
Eavesdropping Attack: Example• tcpdump with promiscuous
network interface– On a switched network, what can you see?
• What might the following traffic types reveal about communications?– DNS lookups (and replies)– IP packets without payloads (headers only)– Payloads
Integrity Attack - Tampering• Stop the flow of the message• Delay and optionally modify the
message• Release the message again
A B
Perpetrator
Authenticity Attack - Fabrication
• Unauthorized assumption of other’s identity
• Generate and distribute objects under this identity
A B
Masquerader: from A
Attack on Availability• Destroy hardware (cutting fiber) or
software• Modify software in a subtle way• Corrupt packets in transit
• Blatant denial of service (DoS):– Crashing the server– Overwhelm the server (use up its resource)
A B
Basic Forms of Cryptography
Confidentiality through Cryptography
• Cryptography: communication over insecure channel in the presence of adversaries
• Studied for thousands of years• Central goal: how to encode information so that
an adversary can’t extract it …but a friend can• General premise: a key is required for decoding
– Give it to friends, keep it away from attackers• Two different categories of encryption
– Symmetric: efficient, requires key distribution– Asymmetric (Public Key): computationally
expensive, but no key distribution problem
Symmetric Key Encryption
• Same key for encryption and decryption– Both sender and receiver know key– But adversary does not know key
• For communication, problem is key distribution– How do the parties (secretly) agree on the key?
• What can you do with a huge key? One-time pad– Huge key of random bits
• To encrypt/decrypt: just XOR with the key!– Provably secure! …. provided:
• You never reuse the key … and it really is random/unpredictable– Spies actually use these
Using Symmetric Keys
• Both the sender and the receiver use the same secret keys
InternetEncrypt withsecret key
Decrypt withsecret key
Plaintext Plaintext
Ciphertext
Asymmetric Encryption (Public Key)
• Idea: use two different keys, one to encrypt (e) and one to decrypt (d)– A key pair
• Crucial property: knowing e does not give away d
• Therefore e can be public: everyone knows it!• If Alice wants to send to Bob, she fetches
Bob’s public key (say from Bob’s home page) and encrypts with it– Alice can’t decrypt what she’s sending to Bob …– … but then, neither can anyone else (except Bob)
Public Key / Asymmetric Encryption
• Sender uses receiver’s public key– Advertised to everyone
• Receiver uses complementary private key– Must be kept secret
InternetEncrypt withpublic key
Decrypt withprivate key
Plaintext Plaintext
Ciphertext
Works in Reverse Direction Too!• Sender uses his own private key
• Receiver uses complementary public key
• Allows sender to prove he knows private key
InternetDecrypt withpublic key
Encrypt withprivate key
Plaintext Plaintext
Ciphertext
Realizing Public Key Cryptography• Invented in the 1970s
– Revolutionized cryptography– (Was actually invented earlier by British
intelligence)• How can we construct an
encryption/decryption algorithm with public/private properties?
– Answer: Number Theory• Most fully developed approach: RSA
– Rivest / Shamir / Adleman, 1977; RFC 3447– Based on modular multiplication of very large
integers– Very widely used (e.g., SSL/TLS for https)
Cryptographic Toolkit
Cryptographic Toolkit
• Confidentiality: Encryption• Integrity: ?• Authentication: ?• Provenance: ?
Integrity: Cryptographic Hashes- Sender computes a digest of message
m, i.e., H(m)– H() is a publicly known hash function
- Send m in any manner- Send digest d = H(m) to receiver in a
secure way:– Using another physical channel– Using encryption (why does this help?)
- Upon receiving m and d, receiver re-computes H(m) to see whether result agrees with d
Operation of Hashing for Integrity
InternetDigest(MD5)
Plaintext
digest
Digest(MD5)
=
digest’
NO
corrupted msg Plaintext
Cryptographically Strong Hashes
• Hard to find collisions– Adversary can’t find two inputs that produce same
hash– Someone cannot alter message without modifying
digest– Can succinctly refer to large objects
• Hard to invert– Given hash, adversary can’t find input that produces it– Can refer obliquely to private objects (e.g., passwords)
• Send hash of object rather than object itself
Effects of Cryptographic Hashing
Cryptographic Toolkit
• Confidentiality: Encryption• Integrity: Cryptographic Hash• Authentication: ?• Provenance: ?
Public Key Authentication• Each side need only to
know the other side’s public key
– No secret key need be shared
• A encrypts a nonce (random number) x using B’s public key
• B proves it can recover x
• A can authenticate itself to B in the same way
E(x, PublicB)
x
A B
Cryptographic Toolkit
• Confidentiality: Encryption• Integrity: Cryptographic Hash• Authentication: Decrypting nonce• Provenance: ?
Digital Signatures• Suppose Alice has published
public key KE
• If she wishes to prove who she is, she can send a message x encrypted with her private key KD
– Therefore: anyone w/ public key KE can recover x, verify that Alice must have sent the message
– It provides a digital signature– Alice can’t deny later deny it non-
repudiation
RSA Crypto & Signatures, con’t
Summary of Our Crypto Toolkit• If we can securely distribute a
key, then– Symmetric ciphers (e.g., AES) offer fast,
presumably strong confidentiality• Public key cryptography does
away with problem of secure key distribution– But not as computationally efficient– Often addressed by using public key
crypto to exchange a session key– And not guaranteed secure
• but major result if not
Summary of Our Crypto Toolkit, con’t
• Cryptographically strong hash functions provide major building block for integrity (e.g., SHA-1)– As well as providing concise digests– And providing a way to prove you know
something (e.g., passwords) without revealing it (non-invertibility)
– But: worrisome recent results regarding their strength
• Public key also gives us signatures– Including sender non-repudiation
• Turns out there’s a crypto trick based on similar algorithms that allows two parties who don’t know each other’s public key to securely negotiate a secret key even in the presence of eavesdroppers
DNS Security
Source: http://nsrc.org/tutorials/2009/apricot/dnssec/dnssec-tutorial.pdf
Root level DNS attacks
• Feb. 6, 2007:– Botnet attack on the 13 Internet DNS root
servers– Lasted 2.5 hours– None crashed, but two performed badly:
• g-root (DoD), l-root (ICANN)• Most other root servers use anycast
Do you trust the TLD operators?
• Wildcard DNS record for all .com and .net domain names not yet registered by others– September 15 – October 4, 2003– February 2004: Verisign sues ICANN
• Redirection for these domain names to Verisign web portal: “to help you search”– and serve you ads…and get “sponsored” search
Defense: Replication and Caching
source: wikipedia
DNS Amplification Attack
580,000 open resolvers on Internet (Kaminsky-Shiffman’06)
DNSServer
DoSSource
DoSTarget
DNS QuerySrcIP: DoS Target
(60 bytes)
EDNS Reponse
(3000 bytes)
DNS Amplification attack: ( 40 amplification )
attacker
Solutions
ip spoofed packets
repli
es
victim
openamplifier
preventip spoofing
disableopen amplifiers
But should we believe it? Enter DNSSEC
• DNSSEC protects against data spoofing and corruption
• DNSSEC also provides mechanisms to authenticate servers and requests
• DNSSEC provides mechanisms to establish authenticity and integrity
PK-DNSSEC (Public Key)
• The DNS servers sign the hash of resource record set with its private (signature) keys
• Public keys can be used to verify the SIGs• Leverages hierarchy:
– Authenticity of nameserver’s public keys is established by a signature over the keys by the parent’s private key
– In ideal case, only roots’ public keys need to be distributed out-of-band
Verifying the tree
stub resolver
Question: www.cnn.com ?
www.cnn.com A ?
resolver
.(root)www.cnn.com A ?
ask .com server SIG (ip addr and PK of .com server)
.comwww.cnn.com A ?
ask cnn.com server SIG (ip addr and PK of cnn.com server)
cnn.com
www.cnn.com A ?
SIG (xxx.xxx.xxx.xxx)
xxx.xxx.xxx.xxx
add to cache
Src.cs.brown.edudns.cs.brown.edu
transaction signatures
slave serverstransaction signatures
42
PKIs and HTTPS
Public Key Infrastructure (PKI)• Public key crypto is very powerful
…• … but the realities of tying public
keys to real world identities turn out to be quite hard
• PKI: Trust distribution mechanism– Authentication via Digital Certificates
• Trust doesn’t mean someone is honest, just that they are who they say they are…
Managing Trust
• The most solid level of trust is rooted in our direct personal experience– E.g., Alice’s trust that Bob is who they say they are– Clearly doesn’t scale to a global network!
• In its absence, we rely on delegation– Alice trusts Bob’s identity because Charlie attests
to it ….– …. and Alice trusts Charlie
Managing Trust, con’t• Trust is not particularly transitive
– Should Alice trust Bob because she trusts Charlie …
– … and Charlie vouches for Donna …– … and Donna says Eve is trustworthy …– … and Eve vouches for Bob’s identity?
• Two models of delegating trust– Rely on your set of friends and their friends
• “Web of trust” -- e.g., PGP– Rely on trusted, well-known authorities (and their
minions)• “Trusted root” -- e.g., HTTPS
PKI Conceptual Framework• Trusted-Root PKI:
– Basis: well-known public key serves as root of a hierarchy– Managed by a Certificate Authority (CA)
• To publish a public key, ask the CA to digitally sign a statement indicating that they agree (“certify”) that it is indeed your key– This is a certificate for your key (certificate = bunch of bits)
• Includes both your public key and the signed statement– Anyone can verify the signature
• Delegation of trust to the CA– They’d better not screw up (duped into signing bogus key)– They’d better have procedures for dealing with stolen keys– Note: can build up a hierarchy of signing
Putting It All Together: HTTPS
• Steps after clicking on https://www.amazon.com
• https = “Use HTTP over SSL/TLS”– SSL = Secure Socket Layer– TLS = Transport Layer Security
• Successor to SSL, and compatible with it– RFC 4346
• Provides security layer (authentication, encryption) on top of TCP– Fairly transparent to the app
HTTPS Connection (SSL/TLS), con’t• Browser (client)
connects via TCP to Amazon’s HTTPS server
• Client sends over list of crypto protocols it supports
• Server picks protocols to use for this session
• Server sends over its certificate
• (all of this is in the clear)
SYN
SYN ACK
ACK
Browser Amazon
Hello. I support(TLS+RSA+AES128+SHA1) or
(SSL+RSA+3DES+MD5) or …
Let’s use
TLS+RSA+AES128+SHA1
Here’s my cert
~1 KB of data
54
Inside the Server’s Certificate• Name associated with cert (e.g.,
Amazon)• Amazon’s public key• A bunch of auxiliary info (physical
address, type of cert, expiration time)• URL to revocation center to check for
revoked keys• Name of certificate’s signatory (who
signed it)• A public-key signature of a hash (MD5)
of all this– Constructed using the signatory’s private RSA key
Validating Amazon’s Identity
• Browser retrieves cert belonging to the signatory– These are hardwired into the browser
• If it can’t find the cert, then warns the user that site has not been verified– And may ask whether to continue– Note, can still proceed, just without authentication
• Browser uses public key in signatory’s cert to decrypt signature– Compares with its own MD5 hash of Amazon’s cert
• Assuming signature matches, now have high confidence it’s indeed Amazon …– … assuming signatory is trustworthy
HTTPS Connection (SSL/TLS), con’t
• Browser constructs a random session key K
• Browser encrypts K using Amazon’s public key
• Browser sends E(K, KApublic) to server
• Browser displays• All subsequent
communication encrypted w/ symmetric cipher using key K– E.g., client can authenticate
using a password
Browser Amazon
Here’s my cert
~1 KB of data
E(K, KApublic)
K
K
E(password …, K)
E(response …, K)
Agreed